Large volumes of oil remain in oil-wet and mixed-wet carbonate reservoirs. By determining whether a formation is water-wet, oil-wet or mixed-wet, the location of the oil can be determined to guide efforts to obtain the oil. Wettability of rocks is a parameter in the characterization of reservoirs and is difficult to measure in situ in the wellbore. There are indications that water-wet sandstones may become oil-wet in the invaded zone of wells drilled with oil-based mud. Knowledge of the wettability of the invaded zone is typically important in the interpretation of Rxo and the nuclear magnetic resonance logs. Wettability of rocks can be determined from core analysis and/or a combination of wireline logs such as resistivity logs, nuclear logs, and nuclear magnetic resonance logs.
However, various methods known in the art for determining wettability involve drawbacks, including substantial amounts of time to obtain a measurement and the need to provide and store radioactive materials.
The electro-kinetic coupling constants of rock samples can be measured accurately in the laboratory. The coupling constants of a large number of water-wet rock samples have already been determined. David B. Pengra, Po-zen Wong, Low frequency AC Electrokinetics, Colloids and Surfaces, 159 (1999) 283-292. However, accurate measurement of the coupling constant in situ in a foundation is more difficult because of the limitations of the measurement geometry. Sensors are typically located only in the borehole; it is usually not practical to isolate a section of rock for measurement. Therefore, the measurements have to be made in an open geometry and are more subject to the uncertainties caused by heterogeneity, anisotropy, bed boundary effects, borehole effects, etc. In principle, the problems are not different from those of any wireline measurements, for example, from the problems of the accurate measurement of resistivity by wireline tools.